Plasmas are used in many sedimentation-, etching-, and layer formation processes.
In plasma reactors that produce a low temperature plasma, either a high frequency voltage of 10 to 30 MHz is applied to two parallel plate electrodes, or microwaves in the GHz region are introduced into a vacuum chamber, in which case more than 500 watts are required.
The most recent attempts involve attempts to produce suitable low-temperature plasmas even under non-vacuum conditions. Such reactors operate with corona discharges or glow discharges. An overview of such plasma generators can be found in Laroussi, Nonthermal Decontamination of Biological Media by Atmospheric-Pressure Plasmas: Review, Analysis, and Prospects, IEEE Transactions in Plasma Science, Vol. 30, No. 4, August 2002, pp. 1409-1415, and also in Schuetze et al. The Atmospheric-Pressure Plasma Jet: A Review and Comparison to Other Plasma Sources, loc. cit., Vol. 26, No. 6, December 1998. The plasma reactions described here are to be used, among other applications, in biological and medical purposes. Aside from the costs encountered with plasma reactors that operate under vacuum, the use of low pressures must often be excluded in this region, so that here the use of an atmospheric pressure plasma is required. Likewise, a treatment of vacuum-sensitive materials, such as certain polymers or sensitive foods, is possible with low-temperature plasmas at or near atmospheric pressures.
So-called plasma needles have already been described in which plasmas produced at a high-frequency electrode in a cylinder through which a process gas flows are suited for use in plasma surgery or for plasma dental treatment, among other applications, see Stoffels et al., Plasma needle: a non-destructive atmospheric plasma source for fine surface treatment of (bio)materials, Plasma Sources Sci. Technol. 11 (2002, pp. 383-388.)
Efforts at reducing the dimensions of plasma reactors for biological and medical, as well as for other purposes, has led to so-called microstructured electrode arrays (MSE) that operate at voltages below 400 V in the 10 MHz-region with structured comb-like electrodes, see Baars-Hibbe et al., Micro-structured electrode arrays: Atmospheric pressure plasma process—characterization and new applications, www.icpig.uni-greifswald.de/proceedings/data/Baars-hibbe 1.
Arrays like so-called MHCD Devices (microhollow cathode discharge) are known, in particular, for applications involving light technology (excimer lasers, fluorescent lamps). In this case microholes are formed in a conducting cathode material. The region remaining between the microholes is coated with a dielectric. An anode lies opposite the entire cathode. The microholes are formed as blind holes for specific applications (U.S. Pat. No. 5,686,789) or they penetrate through the cathode and an underlying substrate material (DE 198 21 244). However, the latter do not operate with a sufficient plasma density for coating tasks.